Known current and voltage transformers for gas-insulated switchgear (GIS) are, for example, based on electro-magnetic induction. These transformers are heavy and voluminous modules and are integral components of a GIS system. The current transformer may be mounted between a disconnector/earthing switch and the circuit breaker. The voltage transformer may be mounted on the opposite side of the disconnector/earthing switch near the bushing. The significant size of the transformers is in contrast to a main goal of GIS (i.e. overall compactness and space savings).
In recent years, more compact electronic and optical sensors have been disclosed. DE 4025911 A1 describes a compact arrangement of optical voltage and current sensors for single-phase encapsulated GIS. The voltage is measured via the piezoelectric effect in disk-shaped quartz crystals positioned on the inside of a metal ring. The pre-assembled sensor module is mounted between two GIS compartments and forms a part of the GIS encapsulation. The piezoelectric deformations of the quartz caused by the electric field around the bus bar are detected with an optical fiber. The module may also contain a mechanical support for an optical fiber coil for magneto-optic current measurement (Faraday effect). The fiber coil may be placed inside or outside the GIS gas compartment (i.e. the gas-filled chamber). Such arrangements include gastight fiber feed-through(s) from the GIS chamber to the outside and involve protection of the fiber against aggressive decomposition products of SF6 produced in electric arcs.
Ref. 1 describes a compact module with combined electronic current and voltage sensors. Here, the current is measured with a Rogowski coil and the voltage is measured with a capacitive divider. The module is again mounted between two GIS compartments.
EP 1710589 A1 discloses a further arrangement of optical current and voltage sensors. Here, the mechanical support structure includes two ring-shaped parts (one electrically conductive, the other non-conductive) which are also mounted between two GIS compartments and again are part of the GIS encapsulation. Between the ring-shaped parts is a groove to accommodate an optical fiber for current sensing. The non-metallic part has a dead end bore (accessible from the outside) that houses a Pockels cell for optical voltage measurement. Again, temperature dependent changes in the dimensions and in dielectric constants may affect the voltage measurement.
The approaches above have in common that retrofit or module exchange is not possible without taking the switchgear from the line and at least partially dismantling it.
WO 2005/111633 discloses a concept for stress-free packaging and orientation of the sensing fiber of a fiber-optic current sensor (e.g., for the precise measurement of high direct currents at aluminum smelters).
U.S. Pat. No. 6,348,786 and U.S. Pat. No. 5,936,395 disclose optical voltage sensors based on electrically poled fibers.
EP 522 303 describes current and voltage sensor comprising a toroidally wound coil for the current measurement as well as a hollow cylindrical sensing electrode for the voltage measurement.